1. Field of the Invention
The present invention relates to an optical device which is used for the production of wavelength controlled semiconductor lasers typical of which are distributed feedback lasers and distributed Bragg reflector lasers, and more particularly to an optical device which is useful for forming on the semiconductor substrate of such a laser a diffraction grating acting as a resonator.
2. Description of the Prior Art
Optical devices of this type heretofore proposed include one which employs a two-beam interference system as disclosed, for example, in N. Tsukada et al., "Grating Formation on Gallium Arsenide by One-step Laser-photochemical Etching" (IEE Conf. Publ. (Inst. Electr. Eng.) No. 227, 1983). With reference to FIG. 2 showing the two-beam interference system, a monochromatic coherent laser beam 1 of highly parallel rays, for example, from a He-Cd laser is divided by a beam splitter 2 into two beams 1-a and 1-b, which are reflected at mirrors 3-1 and 3-2, respectively, and then made coincident on the surface of a screen 4 again at a predetermined angle to produce on the screen 4 interference fringes in the form of periodic vertical stripes. The period P of the interference fringes is given by: EQU P=.lambda./2 sin (.theta./2) (1)
wherein .lambda. is the wavelength of the beam 1, and .theta. is the angle between the beams 1-a and 1-b incident on the screen 4. Thus, if the wavelength is constant, the angle .theta. between the beam 1-a and the beam 1-b needs to be altered in order to vary the period P of the interference fringes. With the system shown in FIG. 2, however, it is then necessary to adjust the mirrors 3-1 and 3-2, and the screen 4 at the same time. The system therefore has the drawback of making the device complex and large-sized.